docs: document the variable-substitution templating language

Document each templated field's variable set as a named data type in
data-types.mdx (Message template, Copy title template, Copy body template,
Workflow input template), each rendering its variables from the JSON schema via a
new TemplateVariablesTable component that reads the x-mergify-template-variables
annotation. The simple-template field type in each action's Parameters table shows
its data-type title and links to it (getValueType: title->anchor, plus 'map of'
for the github_actions inputs map), so feature pages reference the variable set
rather than replicating a table. Adds a Variable substitution concept section with
a Jinja2 migration cheat-sheet, and re-scopes the legacy Jinja2 Template section to
the fields that still use it (post_check, merge).

Change-Id: I530a11f43adb18ff3fdc964199e00d52d105a2b7

Author: kozlek

src/components/Tables/ConfigOptions.tsx

@@ -18,11 +18,20 @@ const valueTypeLinks: { [key: string]: string } = {
 };
 
 const valueTypeFormatLinks: { [key: string]: string } = {
-  template: '/configuration/data-types#template',
+  template: '/configuration/data-types#legacy-template',
   'date-time': '/configuration/data-types#timestamp',
   duration: '/configuration/data-types#duration',
 };
 
+// A simple-template field carries a per-set title (e.g. "Message template"); render
+// the title as the field type, linking to the matching data-types section.
+const simpleTemplateTitleLinks: { [key: string]: string } = {
+  'Message template': '/configuration/data-types#message-template',
+  'Copy title template': '/configuration/data-types#copy-title-template',
+  'Copy body template': '/configuration/data-types#copy-body-template',
+  'Workflow input template': '/configuration/data-types#workflow-input-template',
+};
+
 export type OptionDefinitionRef = string;
 
 export interface OptionDefinitionProperties {
@@ -189,6 +198,19 @@ export function getValueType(schema: object, definition: any): React.ReactElemen
     );
   } else if ('const' in definition) {
     valueType = <HighlightCode>{definition.const}</HighlightCode>;
+  } else if (definition.format === 'simple-template') {
+    // A simple-template field IS a named data type: show its title and link to the
+    // matching data-types section, like Template/Timestamp/Duration.
+    const title: string = definition.title ?? 'simple-template';
+    const titleLink = simpleTemplateTitleLinks[title];
+    valueType =
+      titleLink !== undefined ? (
+        <a color="primary" style={{ textDecoration: 'underline' }} href={titleLink}>
+          {title}
+        </a>
+      ) : (
+        <HighlightCode>{title}</HighlightCode>
+      );
   } else if ('format' in definition) {
     const formatLink = valueTypeFormatLinks[definition.format];
     if (formatLink !== undefined) {
@@ -198,6 +220,13 @@ export function getValueType(schema: object, definition: any): React.ReactElemen
         </a>
       );
     }
+  } else if (
+    definition.additionalProperties &&
+    typeof definition.additionalProperties === 'object'
+  ) {
+    // A map field (e.g. github_actions inputs): show "map of <value type>" so the
+    // templated value type still links to its data-types section.
+    valueType = <>map of {getValueType(schema, definition.additionalProperties)}</>;
   } else {
     valueType = <HighlightCode>{definition.type}</HighlightCode>;
   }

src/components/Tables/OptionsTable.tsx

@@ -1,6 +1,7 @@
 import * as yaml from 'js-yaml';
 
 import configSchema from '../../util/sanitizedConfigSchema';
+import { extractTemplateVariables } from '../../util/templateVariables';
 import Badge from '../Badge/Badge';
 import {
   ConfigSchema,
@@ -50,6 +51,19 @@ export function OptionsTableBase(
             : '';
           const defaultIsMultiline = hasDefault && defaultDump.includes('\n');
           const isDeprecated = Boolean(definition.deprecated);
+          // A simple-template field's description carries an "Allowed variables: …"
+          // list. Once the schema publishes x-mergify-template-variables, the data
+          // type renders a richer <TemplateVariablesTable>, so the duplicate list is
+          // stripped here. Until that annotation is synced in, keep the list — it is
+          // the only variable reference readers have.
+          const rawDescription = (definition as OptionDefinition).description;
+          const hasPublishedVariables =
+            definition.format === 'simple-template' &&
+            extractTemplateVariables(definition).length > 0;
+          const description =
+            hasPublishedVariables && typeof rawDescription === 'string'
+              ? rawDescription.replace(/\s*Allowed variables:.*$/s, '').trim()
+              : rawDescription;
           const id = `${idPrefix}${optionKey}`;
           const href = `#${encodeURIComponent(id)}`;
 
@@ -86,11 +100,11 @@ export function OptionsTableBase(
                   <code>{defaultDump}</code>
                 </pre>
               )}
-              {definition.description !== undefined && (
+              {description !== undefined && (
                 <div
                   className={styles.description}
                   dangerouslySetInnerHTML={{
-                    __html: renderMarkdown(definition.description),
+                    __html: renderMarkdown(description),
                   }}
                 />
               )}

src/components/Tables/TemplateVariablesTable.tsx

@@ -0,0 +1,46 @@
+import configSchema from '../../util/sanitizedConfigSchema';
+import { extractTemplateVariables } from '../../util/templateVariables';
+
+import { ConfigSchema, Def } from './ConfigOptions';
+import { renderMarkdown } from './utils';
+
+interface TemplateVariablesTableProps extends Def {
+  field: string;
+}
+
+export default function TemplateVariablesTable({ def, field }: TemplateVariablesTableProps) {
+  const schema = configSchema as unknown as ConfigSchema;
+  const definition = schema.$defs[def]?.properties?.[field];
+  const variables = extractTemplateVariables(definition);
+
+  // Astro's React SSR rejects a component that conditionally returns null/undefined,
+  // so render an empty fragment (not null) when the field has no published variables
+  // — the graceful state before the engine schema with x-mergify-template-variables
+  // is synced into the docs.
+  if (variables.length === 0) {
+    return <></>;
+  }
+
+  return (
+    <div className="table-wrap">
+      <table>
+        <thead>
+          <tr>
+            <th>Variable</th>
+            <th>Description</th>
+          </tr>
+        </thead>
+        <tbody>
+          {variables.map((variable) => (
+            <tr key={variable.name}>
+              <td>
+                <code>{`{{ ${variable.name} }}`}</code>
+              </td>
+              <td dangerouslySetInnerHTML={{ __html: renderMarkdown(variable.description) }} />
+            </tr>
+          ))}
+        </tbody>
+      </table>
+    </div>
+  );
+}

src/content/docs/configuration/data-types.mdx

@@ -4,6 +4,7 @@ description: The different data types you can find in Mergify configuration file
 ---
 
 import OptionsTable from '../../../components/Tables/OptionsTable';
+import TemplateVariablesTable from '../../../components/Tables/TemplateVariablesTable';
 
 When using templates or conditions, data are made of different types. You will
 find below the different data types that are available and exposed in Mergify
@@ -256,7 +257,67 @@ priority_rules:
     priority: 550
 ```
 
-## Template
+## Templates
+
+Some fields are rendered as templates before Mergify uses them. Most accept
+*variable substitution* (short `{{ name }}` placeholders filled with pull
+request data); a few still use the legacy [Jinja2](#legacy-template) language.
+
+### Variable substitution
+
+Several fields let you insert pull request data using variables:
+
+```text
+Thank you {{ author }} for your contribution!
+```
+
+renders to:
+
+```text
+Thank you jd for your contribution!
+```
+
+when the pull request author login is `jd`.
+
+A variable is written as `{{ name }}` (surrounding spaces are optional). Every
+other character is kept as-is. Each templated field accepts a specific set of
+variables, documented as its data type below.
+
+### Message template
+
+Used by the [`comment`](/workflow/actions/comment),
+[`review`](/workflow/actions/review) and [`close`](/workflow/actions/close)
+message fields.
+
+<TemplateVariablesTable def="CommentActionModel" field="message" />
+
+### Copy title template
+
+Used by the [`copy`](/workflow/actions/copy) and
+[`backport`](/workflow/actions/backport) `title` field.
+
+<TemplateVariablesTable def="CopyActionModel" field="title" />
+
+### Copy body template
+
+Used by the [`copy`](/workflow/actions/copy) and
+[`backport`](/workflow/actions/backport) `body` field.
+
+<TemplateVariablesTable def="CopyActionModel" field="body" />
+
+### Workflow input template
+
+Used by the [`github_actions`](/workflow/actions/github_actions) workflow input
+values.
+
+<TemplateVariablesTable def="GhaActionModelDispatch" field="inputs" />
+
+### Legacy template
+
+:::note
+  Jinja2 is deprecated for fields that support [variable
+  substitution](#variable-substitution) and stops working on 2026-09-30.
+:::
 
 The template data type is a regular string that is rendered using the [Jinja2
 template language](https://jinja.palletsprojects.com/templates/).

src/content/docs/workflow/actions/assign.mdx

@@ -17,7 +17,7 @@ pull requests that require their attention.
 <ActionOptionsTable def="AssignActionModel"/>
 
 As the list of users in `add_users` or `remove_users` is based on
-[templates](/configuration/data-types#template), you can use, e.g.,
+[templates](/configuration/data-types#legacy-template), you can use, e.g.,
 `{{author}}` to assign the pull request to its author.
 
 :::caution

src/content/docs/workflow/actions/backport.mdx

@@ -32,19 +32,6 @@ strings.
 
 <ActionOptionsTable def="BackportActionModel" />
 
-As the title and body are templates, you can leverage any pull request
-attributes to use as content, e.g., `{{author}}`.
-
-Note that the `commits` attribute here will be the list of cherry
-picked commits.
-
-On top of that, you can also use the following additional variables:
-
-- `{{ destination_branch }}`: the name of the destination branch.
-
-- `{{ cherry_pick_error }}`: the cherry pick error message if any (only
-    available in body).
-
 <BackportLimitations />
 
 ## Examples
@@ -101,10 +88,11 @@ In this configuration, a pull request is backported when it has the label `backp
 Then, when the backport is created and passes the check named
 `continuous-integration`, it will be automatically merged.
 
-### Implementing `-x` option
+### Including the cherry-picked commits
 
-If you are used to the `-x` option of `git cherry-pick` that includes which
-commits has been cherry-picked, you can implement the same thing with Mergify:
+To record which commits were cherry-picked (similar to the `-x` option of `git
+cherry-pick`), include their SHAs in the backport body with the
+`{{ cherry_picked_commits }}` variable:
 
 ```yaml
 pull_request_rules:
@@ -116,9 +104,7 @@ pull_request_rules:
         body: |
           {{ body }}
 
-          {% for c in commits %}
-          (cherry picked from commit {{ c.sha }})
-          {% endfor %}
+          {{ cherry_picked_commits }}
         branches:
           - stable
 ```

src/content/docs/workflow/actions/copy.mdx

@@ -34,19 +34,6 @@ request will be copied. The branch names should be specified as strings.
 
 <ActionOptionsTable def="CopyActionModel" />
 
-As the title and body are templates, you can leverage any pull request
-attributes to use as content, e.g., `{{author}}`.
-
-Note that the `commits` attribute here will be the list of cherry
-picked commits.
-
-On top of that, you can also use the following additional variables:
-
-- `{{ destination_branch }}`: the name of the destination branch.
-
-- `{{ cherry_pick_error }}`: the cherry pick error message if any (only
-    available in body).
-
 <CopyLimitations />
 
 ## Examples

src/content/docs/workflow/actions/github_actions.mdx

@@ -40,7 +40,8 @@ following rule.
 
 Here, the `hello_world_workflow.yaml` workflow accepts two
 inputs, which are defined as `name` and `age`. The `dynamic_workflow.yaml`
-takes the [template](/configuration/data-types#template) input `author`.
+takes the `author` input, set with [variable
+substitution](/configuration/data-types#variable-substitution).
 
 ```yaml
 pull_request_rules:

src/content/docs/workflow/actions/merge.mdx

@@ -247,7 +247,7 @@ commit_message_format:
 ### Migrating from `commit_message_template`
 
 The legacy `commit_message_template` setting is a
-[template](/configuration/data-types#template) that renders the entire
+[template](/configuration/data-types#legacy-template) that renders the entire
 commit message. `commit_message_format` covers the common patterns
 declaratively, with predictable output and no template engine.
 

src/content/docs/workflow/actions/post_check.mdx

@@ -74,7 +74,7 @@ status of a pull request based on Mergify's evaluation.
 <ActionOptionsTable def="PostCheckActionModel" />
 
 As the `title` and `summary` are
-[templates](/configuration/data-types#template), you can benefit from any [pull
+[templates](/configuration/data-types#legacy-template), you can benefit from any [pull
 request attributes](/configuration/conditions#attributes-list), e.g. `{{author}}`,
 and also these additional variables: